1. Field of the Invention
This invention relates to a process for the catalytic hydrogenation of liquid fatty acid triglycerides for the simultaneous recovery of fatty alcohols and C.sub.3 diols in the presence of gaseous hydrogen and hydrogenation catalysts under pressures of from 50 to 300 bar and at temperatures of from 160.degree. to 250.degree. C.
2. Description of the Related Art
A process of this type is known in German patent application No. DE 36 24 812. This process uses catalysts based on copper chromite with which it is possible to control the direct hydrogenation of triglycerides to fatty alcohols with high activity and selectivity under comparatively moderate reaction conditions in such a way that propane-1,2-diol is obtained as valuable secondary product and may be used for the production of alkyd or polyester resins and for many other applications.
Since these and also other hydrogenation catalysts catalyze the degradation of the valuable product, propane-1,2-diol, to secondary products and the degradation of the fatty alcohols to paraffins under hydrogenation conditions with an increase in the residence time of the reaction mixture, the hydrogenation reaction can only be stopped at the stage of the desired products by maintaining special process conditions. However, it is not known from DE No. 36 24 812 how the catalytic hydrogenation of triglycerides can be accomplished on an industrial scale.
Catalytic reactions of the type in question are often carried out in fixed-bed reactors in which the fluid phases are introduced into the reactor as trickle phase. Reactors of this type are described, for example, in Ullmanns Enzyklopadie der technischen Chemie, 4th Edition, Vol. 3, pages 500 et sec. In these reactors, the reaction takes place under adiabatic conditions, i.e. the temperature increases along the catalyst packing during the mostly exothermic reactions. However, since the selectivity of most catalysts is greatly dependent on temperature, changes in the reaction mechanism can occur in consequence of the changes in temperature in the reactor so that mostly unwanted secondary reactions take place. In addition, excessive temperatures can cause irreversible damage to the catalyst.
To limit the increase in temperature in the reactor, it is known that the gas phase may be passed in a large excess through the reactor or several reactors arranged one behind the other with intermediate cooling may be used. However, this method of temperature control is unsatisfactory for the hydrogenation of fatty acid triglycerides because the unwanted secondary reactions cannot be prevented in this way and the catalysts can suffer losses of activity through recrystallization and structural changes if certain temperatures are exceeded.
It is also known that catalytic reactions of the type in question can be carried out in an isothermally operated tube bundle reactor (Chemie-Technik, Vol. 4 (1975, No. 12, pages 439-441). In the so-called Bayer cold hydrogenation process, catalytic hydrogenations are carried out at low temperatures under substantially isothermal conditions so that the catalyst is not subjected to any variations in temperature and there is no danger of overheating. However, the reaction described in this literature reference is an extremely non-critical hydrogenation reaction, i.e. only one valuable product is targeted and the reaction product is not subsequently reacted to the same extent, if at all, so that if necessary unreacted components may be recycled; even an extended residence time under hydrogenation conditions does not harm the product.
Accordingly, the object of the present invention is to provide a process for the direct catalytic hydrogenation of triglycerides to fatty alcohols and propanediol under industrial conditions with yields of propanediol of more than 80% of the theoretical and with a content of paraffins of no more than 0.5% of the theoretical.